It is frequently desirable to analyze the motion of an object moving in two dimensions. That is, to determine its speed, acceleration, or location on a real time basis. As an example, the side view film of automobile crash tests is commonly seen, but for valid testing, much more precise measurements of the vehicle and the characteristics of its motion along a constraint path may be required. Another field that requires precise measurements of a moving object's position, velocity and acceleration is that of motion pictures—in which photographic, or electronic recordings of a scene are captured.
Motion picture visual effects often require the photography of multiple individual images which must be combined to form one integrated image. If that composite image is intended to appear as from a spatially moving viewpoint, then all the aforementioned individual images may also be required to be imaged from that identical moving viewpoint. The process of repeatably imaging a physically moving viewpoint forms the basis for what is commonly known as “motion-control” photography, in which motorized devices move the viewpoint in predetermined or recordable trajectories or spatial pathways. In some cases, a viewpoint's spatial path may need to be recorded for other purposes, such as acquiring the data necessary for computer generated imagery, a process commonly referred to as “motion capture”.
One of the more difficult of the types of motion to accurately measure has been the two dimensional movement of a camera cart, or “dolly”, along the track it rides upon. Factors contributing to this difficulty include the variable and potentially long distances—well over one hundred feet in some cases—over which the dolly may travel, and that some sections of track may need to be curved. The track itself is not unlike a small railroad track, with about a two foot spread between rails, the rails being typically about two inches high. The tops of the rails are convex, and the dolly's wheels are concave, or in some cases “V” shaped to provide a wedge fit to the rail.
Several types of approach have been taken in the past to attempt accurate measurement between dolly and track. The approach most frequently taken consists of measuring the motion of a motor driving the dolly. Whether the drive mechanism to which the motor is attached is based on mechanical principles such as rack-and-pinion gearing, timing belt, friction drive or other such means, these drives may ensure repeatable measurements at the drive itself but, due to issues of backlash, compliance, or slippage, measurements taken at the motor in such mechanisms may not necessarily provide an accurate representation of movement between the dolly and the track. In cases where a drive was unnecessary or undesirable, various devices have been used for measurement between dolly and track. A simple friction-contact rubber-wheeled capstan on a common shaft with an encoder, mounted to the dolly and rolling in contact with the track has commonly been used but, due to slippage between capstan and rail, requires frequent recalibration to some reference point on the track. Another approach has been to attach one end of a cable to the dolly, with the cable's opposite end wound on the shaft of a rotary encoder attached to one end of the track, while maintaining constant tension on the cable. Dolly movement in this configuration results in a corresponding encoder shaft rotation, allowing a resultant encoder signal to be generated. Measurement inaccuracy in such applications can result due to elasticity in the cable, and applications requiring the use of curved track sections have prohibited use of this approach.
In film and video production, minimizing setup time is considered to be of great importance. These aforementioned track drive measurement implementations share common drawbacks in such an environment: they may require considerable time to assemble mechanical and electronic components, to align these components, and to calibrate the system to known reference points and in meaningful units of measurement.
The goal of this new approach is a two dimensional measurement method that, among other advantages in the field of motion picture production, is quick and easy to use, is capable of maintaining its measurement accuracy in situations where there may be backlash in the drive, can be used with combinations of straight and curved track, works with or without an external drive, and may be used with most existing track and drive systems. Of course in the general sense of recording the movement of an object other than a camera, many of the same advantages can be realized.
This new method comprises placing a sensor on the moving object (in the case of a camera, the dolly) and affixing markings along the surface over which the object moves (in the case of a dolly, some surface of the track). Naturally, in other applications, the surface could move, and the object remain stationary. The sensor detects and decodes the markings to produce information about the object's motion in real-time, whether that information is position, velocity or acceleration. The information may be used simply to record the motion, or even used to close a servo loop. This approach works well whether an external force moves the object, or when internal power drives the object.